I had a 2004 RX-8 that I really loved to drive but hated to own. I bought it new and it had more recalls than any car I ever owned, at least 10. I finally got sick of it and traded it. The rotary was smooth, the car was over-built (lots of extras) but again, too much trouble to own. My advice to the OP is to try and get out of it ASAP. Rocketman
Sorry OP, I didn’t speak to your present problem. If you can afford a newer car, go to a Mazda dealer and look around. Try to make a deal and then tell them you have an RX-8 to trade, they can fix it a lot cheaper than you and the car market is good for buyers right now (I just bought a new car in September). I traded my RX-8 in at a Mazda dealer and got top dollar for it on a new (piston) Mazda. Get out of the RX-8 now, I kept mine for too long, lots of problems. Rocketman
@"the same mountainbike" no worries, and I appreciate the willingness to help out!
@ricketman this is my girlfriends car and I am trying to get it back up and running so we can do just that; get rid of it and find something more reliable and suitable to haul around the kids.
Edit: won’t be taking this to trade in at the dealership. Just trying to get it to run.
Good luck heavymetal, I finally just got tired of the problems. The Mazda dealer gave me the most money for a trade, I hope that you get it going and move it along to someone else. Nice cars but problematic. Rocketman
So I called duralast customer support and they said that what I have is the right part, the guy I spoke with had me do a few voltage tests which I did and everything was what it was supposed to be. He said to try and swap it out again or get the part from somewhere else. The dealership did not have one in stock I could compare it to. I will have to order it online.
Here are some pictures as promised.
obligatory banana for scale.
You’re sure the engine isn’t turning over while cranking? I would think a non-engaging starter would make a higher pitched whine. Have you run the new starter on the bench?
It’s hard to tell in photos, as excellent as they are, but it appears that the alignment (registration) pin on the mounting surface is about at the centerline of the ring gear. The corresponding registration hole in the cast housing looks too far from the centerline of the ring gear to allow the gears to mesh.
The centered bolt hole (top photo, across from the banana, is about tangential to the starter gear circumference, while the beveled bolt hole on the mounting surface looks too far away from the ring gear centerline to allow engagement with the depth that the starter gear looks like it protrudes into the mounting surface.
Are you sure you’re getting tooth engagement when the starter gear slides over?
It doesn’t sound like the teeth touch at all. I will slap it back on tomorrow or the next day and record the sound again next to the starter and under the engine and post it
About all I can suggest is measuring from the center of the dowel pin to the center of the flywheel teeth and then doing the same on the starter motor by measuring from the pin hole to the center of where the starter drive gear teeth are when engaged. It should be very close.
From the pics something doesn’t look quite right to me but that could just be the perspective.
Maybe time to rig up a cartridge start system like some aircraft use… 
Can you energize the bendix assembly to get the gear to slide over and “dry fit” the assembly only the mounting surface? Perhaps that’ll clarify once and for all whether the teeth are engaging. Two 6VDC lantern batteries in series wouldn’t provide sufficient current to drive anything, but they should energize the solenoid. I admit, I’ve never tried this, but I wouldn’t be afraid to. I can’t imagine how it would hurt anything.
@the same mountainbike I have a jumpbox that I can use to push out the plunger or bendix or whatever its called. It does pop out. Are you wanting me to do this when the starter is on the car?
Another idea is to bench-measure how far it pops out, then figure out some kind of test fixture you can use on the car to see if that amount of pop out from the starter motor mounting surface will engage the flywheel.
I’ve done the pop out experiment with my Corolla starter on the bench before, using a car battery as the power source, and got best results (reliable connections with less sparks) by using a 30 amp/12 volt/ 30 ohm coil relay, rather than trying to jump the “start” connection directly. I bought the relay at Radio Shack for around $5.
It sure looks as if the gears would not even be close to meshing.
Measuring the pitch of both gears, starter and flywheel, might make sense.
Heavymetal, I was unaware that you were able to do this. My thought is to simply check to see how/if the gears are engaging when the hosing is mounted and the gear slid out into its engaged position. If you can do that manually without the motor on that’s even better.
For the record, the “bendix” is the generic term commonly used to describe the solenoid/lever/contacts mechanism that engages the starter gear and enables the starter circuit. These mechanism’s aren’t actually all technically Bendix mechanisms any more than all tissues are Kleenex, but the name has evolved into a generic term.
You could place the starter against metal on the car and run a jumper wire to the small terminal on the solenoid. The starter gear should kick out all the way although it won’t rotate.
If the gear does kick out all the way you could use various reference points on the starter flange and bell housing flange and eyeball whether that gear should be meshing or not.
Just curious, but what led to the starter issue? Random problems with it, a sudden failure, grinding, etc, etc?
This is somewhat unusual and I hate to even mention it but not any chance the flywheel attachment bolts or the crank broke and the starter is actually rotating the flywheel but not the engine?
Just throwing the other world stuff out since you have such an unusual problem here. 
In the video the tachometer needle drops when the starter stops. The gauge may be reacting to voltage change but I suspect the crankshaft is turning. Before you install the starter try spinning the ring gear by hand, does it turn easy? After you install the starter observe the pulleys while cranking the engine.
BTW, I had a recall and change of my starter one time on my RX, you may want to check if it had a different starter replaced at some time. Or maybe Mazda will do it for free. Worth a check. Rockteman
Here is a video of the sound of the starter, after having my gf try and start the car while I was under the hood, I saw that the belts were moving so that would mean the starter is engaging… right? anywho I am still stumped as to why the new starter isn’t starting the car. The battery is brand new and kept it on a maintainer.
That puts things in a different light. Your starter motor is working. Now you’re dealing with a worn out engine (not likely…), lack of spark, or lack of fuel.
Take a can of aerosol carb cleaner and spray a healthy shot into the intake tract. If it starts and runs for a few seconds then you have a lack of fuel issue due to a bad fuel pump or the electronic controls involved in making the pump operate.